1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of forming a field oxide film for element isolation.
2. Description of the Related Art
Local oxidation of silicon (LOGOS) is widely used as an element isolation technique. A field oxide film Is required to be thicker than a gate oxide film or other films. A fast oxidation speed is required for a field oxide film so as to improve throughput. Wet oxidation has been therefore used conventionally.
In order to further speed up the oxidation speed, it is preferable to use as high an oxidation temperature as possible. However, high temperature oxidation causes nitrogen atoms in a silicon nitride (hereinafter represented as SiN) film used as an oxidation mask to react with hydrogen atoms in moisture and generate ammonium. Ammonium diffuses in a silicon oxide film formed under the SiN film as a strain absorbing layer and reaches the silicon substrate surface.
On the silicon substrate surface, nitrogen atoms in ammonium react with the silicon substrate and form SiN. In this manner, SiN is formed on the surface of the silicon substrate near at the outer periphery of the SiN film used as the mask, i.e., near at the outer periphery of the field oxide film. Since this SiN is formed under the oxide film used as the strain absorbing layer, it can't be removed by the process of removing the SiN film used as the mask.
At the later process of forming a gate oxide film by thermal oxidation, this SiN functions as a mask so that a stripe region called a white ribbon is left near at the outer periphery of the field oxide film without being subjected to thermal oxidation. This stripe region lowers a breakdown voltage of the gate oxide film and deteriorates an element performance. In order to prevent generation of a white ribbon, a lower oxidation temperature is desired.
Conventionally, wet oxidation has been performed at a temperature of about 900.degree. C. in order both to raise an oxidation speed and to prevent generation of a white ribbon.
A lateral oxidation speed is generally faster than a vertical oxidation speed at a temperature of about 900.degree. C. Therefore, a bird's beak at the end of the field oxide film creeping under the SiN film and moving it up is likely to become long.